Broadband aircraft foil antenna



June 12, 1962 B, A. s. JOSEPHSON ,09

BROADBAND AIRCRAFT FOIL ANTENNA Filed Jan. 9, 195a FIG I FIG 2 FIG 3 FIG 4 Inventor:

United States The antenna according to this invention is suitable for use at short radio Waves. Its radiation pattern is essen-- tially the same as that of a rod antenna, ie it is essentially omnidirectional in a plane perpendicular to its length direction. By combining two or more driven or parasitic antenna elements designed according to the invention, directional properties can be achieved.

Priority is claimed under Rule 55 from the date of the Swedish application, of which a certified copy is in the co-pending application No. 709,432, now Patent No. 2,994,876.

The antenna is particularly suited for use as an external antenna on an aircraft.

On modern aircraft the antenna problem becomes more and more difficult to solve, due to the increasing number of antennas needed, the severe requirements on their mechanical strength and stream-line shape at high speeds, the claims for an accurate match to the feeding line within large frequency bands, and so forth. The use of external antennas on an aircraft has many drawbacks, such as increased drag, influence on the stability of the airplane, risk of ice formation and risk of damage. The use of suppressed antennas and slot antennas would solve some of these difficulties, however, this is often not feasible, because the necessary space within the skin of the aircraft might not be available.

Accordingly it is very desirable that appropriate external antennas be devised, and above all one must observe that these get good stream-line shape, good broadband impedance match and that they be of simple and inexpensive design in view of the need for exchange when damaged.

The antenna according to the invention fulfills these requirements.

In an antenna according to the invention the radiating elements consist of the wider conductors of a strip-line system, wherein the conductors have the form of very thin metal foils of different width, applied to thin dielectric plates. Such strip-lines are previously known and used instead of conventional transmission lines, e.g. for feeding antennas and for building up various transmission line components Within the microwave technique, such as joints, directional couplers, transformers and the like.

In an antenna according to the invention plane conductors of this type serve simultaneously as radiating elements and as impedance correcting elements for achieving a good match to an external feeding cable. Such an antenna is in the following called a strip-line antenna. Within the frequency bands of main interest here (higher than about 100 MHZ.) the penetration of energy in good conductors is small compared to the thickness of thin foils. Thus one side of a metal foil may be used as radiating part (viz. carry antenna currents) while the other side of same foil is used for such purposes as feeding and impedance matching by constituting part of line elements built into the antenna.

There are two particular advantages arising from the employment of a strip-line arrangement in an antenna intended for external use on a aircraft: Without extra space requirements and mechanical complexity impedance correcting elements may be built into the antenna, thus making other matching elements, such as a parallel stub with in the skin of the aircraft, unnecessary, which facilitates atent the placing of the antenna on the aircraft. The other advantage is that the complete antenna conductor arrangement, including matching elements, can be made very thin, thus enabling" the embedding of this structure in a dielectric covering of good aerodynamic shape. The necessity of a thin structure inside the covering will be clear when considering that, on high speed aircraft, the cross section of an external antenna must have a ratio between Width and length which does not exceed about 0.05.

The idea of the invention is exemplified in the following illustrations. I

FIGURES 1 and 2 show an antenna having a pair of dielectric plates and metal foils in three planes, FIGURE 1 being a sectional view along the arrows of FIGURE 2. FIGURES 3 and 4 show a strip-line antenna embedded in a dielectric covering, FIGURE 3 being a sectional view along the arrows of FIGURE 4.

In the embodiment of the invention shown in FIG- URES l and 2 the items 6 and 7 are two thin dielectric plates, contiguous to one another and serving as mechanical support to the conductors of the antenna. The conductors consist of metal foils arranged in the three surface planes of the dielectric plates. 'Iwo equiform, relatively wide foils 3 and 4 are applied to the outer surfaces of the dielectric plates and are through a suitable method connected to the counterpoise along their lower edges. A third relatively wide metal foil 5 is applied to the inner surface of one of the dielectric plates in a position above, seen. from the counterpoise, the foils 3" and 4. The narrow foils or strips 1 and 2 are located in the same plane as the foil 5 At the antenna base a means is provided for connecting an external coaxial feeding cable to the antenna. The strip -1 runs from the center conductor of said connecting means to the foil 5. From the point where strip 1 is connected to foil 5 the currents at strip I spread out over substantially the whole surfaces of foil 5 and these surface currents give rise to a radiation field. Associated with the currents at both sides of the strips 1 and 2 flow currents at the insides of the wide foils 3 and 4, these currents being confined to approximately the projected areas of the strips on the foils, and having together the same magnitude but opposite direction as the total currents on the strips. At the point where strip .1 passes the edges of the foils 3 and 4 the inside currents at these foils turn round the edges and spread out over the outer surfaces, thereby contributing to the radiation field. Thus the radiating portions of the antenna are the whole of foil 5 and the outsides of foils 3 and 4. The actual driving point of the antenna is the gap where strip 1 passes the edges of foils 3 and 4, the antenna thereby being driven between foil 5 and the foils 3 and 4 in parallel. The strips 1 and 2 constitute, in conjunction with portions of the foils 3 and 4, a built-in transmission line element for driving the antenna and improving its impedance characteristics. By a suitable choice of the dimensions of these foils and of point of connexion between them a still more broadband match to the external feeding cable can be achieved than would be possible by connecting a constant impedance cable direct to the actual driving point referred to above.

Another method of arranging the radiating and impedance matching conductors in a broadband antenna which according to the invention comprises two equiform, dielectric plates contiguous to one another and metal foils applied to the three surface planes is the following. At each outside of the two dielectric plates a relatively wide foil is applied. The lower extremity of each foil, seen from the counterpoise, is connected to the center conductor of a receptacle disposed at the antenna base for connecting a coaxial feeding cable to the antenna. A narrow foil is applied to one of the dielectric plates at the side facing 3 the other plate. This narrow foil is at one end connected to the counterpoise and is so positioned as to be covered by both the wide foils. The narrow foil in conjunction with the wide foils forms a built-in transmission line element which by a suitable choice of dimensions acts to improve the impedance characteristics of the antenna.

FIGURES 3 and 4 illustrate how the internal parts of a strip-line antenna can be embedded in a dielectric covering, which can be given the necessary mechanical strength and an external shape having good aerodynamical properties. The conductor system of a strip-line antenna is by its suitable and simple form well adapted for this procedure, and can, together with its receptacle for a feeding cable, be manufactured in a complete unit, which can be tested electrically before it is embedded in the covering. The covering, however, affects the electrical properties (mainly the impedance) of the antenna, and this must be taken into consideration when giving the final shape of the conductor system.

The covering, suitably, is made of a plastic, reinforced by glassfiber cloth. The cloth should be laid as shown in FIGURE 4, so as to avoid joints at the antenna base.

What is claimed is:

1. External broadband aircraft antenna, comprising two thin dielectric plates contiguous to one another, three separate conductors confined one to each of the three parallel surfaces defined by said plates and consisting of an inner foil intimately applied to one of the inner surfaces of said plates and two outer foils intimately applied to the outer surfaces thereof in such position that the perpendicular projections of all said foils on one of said surfaces include a common area, said metal foils being dimensioned and adapted to be connected to a driving source so as to enable portions thereof to act as the radiating elements of the antenna and to enable the portions of said foils projected on said common area to confine an electromagnetic field, whereby said other projected portions act as non-radiating transmission line elements.

2. Antenna in accordance with claim 1, comprising a connecting means disposed at the antenna base for connecting a coaxial feeding cable, wherein said outer foils have a large width compared to the combined thickness of said dielectric plates and are disposed along their lower edges to be electrically connected to the skin of the aircraft, and wherein said inner foil includes a narrow portion at one end connected to the center conductor of said connecting means and at the other end broadening to a portion having a large width compared to the combined thickness of said dielectric plates and extending substantially above said outer foils.

3. Broadband antenna disposed to be mounted on a counterpoise, comprising two equiform thin dielectric plates facing and contiguous to one another, three separate conductors confined one to each of the surfaces defined by said plates and consisting of two equiform relatively wide metal foils and at least one narrow metal foil, a connecting means disposed at the antenna base for connecting an external coaxial feeding cable to the antenna, said narrow foil being disposed in the intermediate plane of said dielectric plates, said wide foils being applied to respective outsides of said dielectric plates, facing one another and covering the narrow foil along substantially the whole length of same whereby said narrow foil constitutes one a part of a built-in non-radiating transmission line, the other part of which being constituted by portions of said equiform wide foils, and, wherein at least one of said foils is connected to the counterpoise and at least one is connected to the center conductor of said connecting means.

4. Antenna in accordance with claim 3 comprising a third relatively wide metal foil disposed in the intermediate plane of said dielectric plates and from the counterpoise seen located above said two equiform metal foils, these two foils along their lower edges being connected to the counterpoise, one end of said narrow foil being connected to the center conductor of said connecting means disposed at the antenna base and the other end being connected to said third wide foil, whereby said built-in transmission line drives the antenna between said two equiform wide foils in parallel and said third Wide foil.

5. Antenna in accordance with claim 4, comprising an auxiliary narrow metal foil disposed in the intermediate plane of said two dielectric plates in such a position as to be covered by said two equiform wide foils, said auxiliary foil at one end being connected to a suitable point of said first narrow foil, thereby forming a built-in parallel stub, the length and width of the auxiliary foil being so chosen as to improve the impedance characteristic of the antenna as seen from the external feeding cable.

References Cited in the file of this patent UNITED STATE PATENTS 2,344,884 Kirkland Mar. 21, 1944 2,412,249 Brown et al Dec. 10, 1946 2,505,751 Bolljahn May 2, 1950 2,612,606 Wehner Sept. 30, 1952 2,797,390 Kostriza et a1. June 25, 1957 FOREIGN PATENTS 886,168 Germany Aug. 3, 1953 1,050,583 France Sept. 2, 1953 

